Flow Rate From Displacement Unit Piston Position

ABSTRACT

A method to detect flow rate from a displacement unit entailing locating the displacement unit in a fluid path, monitoring a piston position in the displacement unit for a stroke direction change, starting a timer upon a stroke direction change, monitoring at least one check valve position for an alteration of position, stopping the timer upon the alteration of position, calculating a time between the stroke direction change and the alternation of position, calculating a volume of the displacement unit and calculating the flow rate from the displacement unit by dividing the volume of the displacement unit by the calculated time.

CROSS-REFERENCE TO RELATED APPLICATIONS

None.

FIELD OF THE INVENTION

Aspects of the disclosure relate to formation testing. More specifically, aspects of the disclosure relate to downhole formation testing methods that have accurate measurements for flow rates over a large dynamic range.

BACKGROUND INFORMATION

Testing of geological formations is an important part of the modern oil and gas industry. Formations are tested by operators and engineers to determine the constituent parts of hydrocarbons. Once testing is completed to the satisfaction of engineers, completion operations may take place to optimize the hydrocarbon withdrawal from the wellbore.

Many formation testers use a reciprocating piston pump to move formation fluid from formation to the borehole or vice versa. Several different types of drive mechanism (e.g. motor screw, motor hydraulic, linear motor) can be used (FIG. 1A and FIG. 1B) to drive a displacement unit piston. The flow in and out of the displacement unit is routed through a number of check valves. Reciprocating piston pumps can generate flow rates covering a large dynamic range with either a single or multiple piston pumps. Due to this large dynamic range, accurate measurement of the achieved flow rates over the entire range is difficult with a single sensor. Adding to the flow rate measurement complexity is the fact that with compressible fluids the mass flow rate on the low pressure side of the pump will be different to the mass flow rate on the high pressure side of the pump and the fact that formation testers may have multiple pumps on multiple flowlines. An alternate method of determining downhole flow rates is desired.

SUMMARY

A method to detect flow rate from a displacement unit is provided entailing locating the displacement unit in a fluid path, monitoring a piston position in the displacement unit for a stroke direction change, starting a timer upon a stroke direction change, monitoring at least one check valve position for an alteration of position, stopping the timer upon the alteration of position, calculating a time between the stroke direction change and the alternation of position, calculating a volume of the displacement unit and calculating the flow rate from the displacement unit by dividing the volume of the displacement unit by the calculated time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a cross-sectional view of a reciprocating piston pump used in downhole operations.

FIG. 1B is a schematic view of a screw drive coupled to a displacement unit.

FIG. 2A is a cross-section of a displacement unit at an end of stroke condition.

FIG. 2B is a cross-section of a displacement unit at an upward stroke condition and high pressure in an upper cavity.

FIG. 2C is a cross-section of a displacement unit at an upward stroke condition and lower pressure in the upper cavity.

FIG. 2D is a cross-section of a displacement unit in an upward stroke condition and high pressure lower cavity.

FIG. 3 is a graph of volumetric flow rate in a displacement unit between each stroke end.

FIG. 4 is a flowchart for a first methodology for calculating a flow rate from a displacement unit piston.

FIG. 5 is a flowchart for a first methodology for calculating a flow rate from a displacement unit piston.

DETAILED DESCRIPTION

The FIGS. 1A and 1B show an example of a hydraulically driven reciprocating piston pump (FIG. 1A) and a motor screw drive reciprocating piston pump (FIG. 1B). Past efforts to determine flow rate of hydraulically driven reciprocating pumps have relied on inferring the hydraulic oil rates and translating these rates to formation fluid rates.

In an example embodiment, flow rates are determined from knowledge of the piston position. Depending on the drive mechanism, piston position may also be measured by a dedicated sensor or derived from the motor screw position.

With accurate piston position and incompressible fluids, flow rate is calculated through Equation 1. This flow rate is independent of pressure.

$\begin{matrix} {{{Flow}\mspace{14mu} {rate}\mspace{14mu} Q} = {{Flow}\mspace{14mu} {Area} \times \frac{{Piston}\mspace{14mu} {translation}\mspace{14mu} {distance}}{{Piston}\mspace{14mu} {translation}\mspace{14mu} {Time}}}} & {{Equation}\mspace{14mu} 1} \end{matrix}$

Fluids pumped by formation testers have some compressibility. Fluid is drawn into the piston pump at a pressure below formation pressure and the fluid is forced out of the pump at a pressure that is at or above hydrostatic pressure. The mass flow rate at the intake side will therefore be different from the mass flow rate at the pump output side. FIG. 2 demonstrates flow in and flow out of the displacement unit. A hydraulically driven system is shown, but the concept is similar for other drive systems.

In FIG. 2A, the downward movement of the piston reaches the end of the stroke. Some dead volumes are inevitably left in the upper cavity. Dead volume includes the flowline volume between the displacement unit and the check valve (not shown). The upper cavity fluid is at hydrostatic pressure (high pressure). The lower cavity contains fluid at or below formation pressure (low pressure). Referring to FIG. 2B, the piston changes direction. Before fluid moves into the upper cavity, the fluid needs to be depressurized. Similarly, no fluid will flow out of the lower cavity until the fluid therein is pressurized. Referring to FIG. 2C the upper cavity is de-pressurized and fluid flow starts. Referring to FIG. 2D, the lower cavity is pressurized and fluid flow starts.

As previously provided, the difference in flowing pressure at the intake and output sides of the piston pump creates difference in mass flow rate. However, once flow starts the volumetric flow rate will be the same on both sides. FIG. 3 shows the volumetric flow rate versus time. When the piston cavities have been compressed/decompressed (in FIG. 2 this is the time between FIG. 2D and the stroke end (FIG. 2A) the volumetric flow rate is independent of the flowing pressure and the flow rate at the intake and output of the reciprocating piston pump can be calculated simply by equation

FIG. 3 schematically depicts the volumetric flow rate at the intake side of the pump and at the output side. A downhole fluid density sensor placed at the relevant pressure side of the pump will allow computing mass flow rate from the volumetric flow rate. For multi-phase flow the proportions and densities of each phase must be known.

To calculate the time between each stroke-end and the start of flow into and out off the displacement unit will require additional information.

Several methods are provided.

In a first example embodiment, for fluid to flow, the sand face -pressure is greater than pressure at the displacement unit inlet. Such difference in the pressure at the sand face to the displacement unit inlet will cause free fluid to flow into the inlet. Thus, measurement of the sand face flowing pressure by a pressure gauge at the tool to formation interface would indicate fluid flow. For fluid flow at the output side of the displacement unit, if pressure at the displacement unit output is greater than or equal to hydrostatic pressure, then fluid flow is present. Thus, in a non-limiting embodiment, a tool is placed into a downhole position that is desired to be measured. A pressure is read regarding a sand face flowing pressure at the tool to formation interface at the input side. A pressure reading may also be read at the displacement unit output side. Knowing the pressure at the individual openings (the inlet and the outlet side), the individual volumes entering and leaving the displacement unit may be calculated.

In another embodiment, with a known compressibility, constant temperature and constant mass, the change in volume (AV) required to compress/decompress the displacement unit can be calculated through Equation 2, where the constant C represents compressibility and the variables V & P denote Volume and Pressure:

$\begin{matrix} {C = {{- \frac{1}{V}} \cdot \frac{\partial V}{\partial P}}} & {{Equation}\mspace{14mu} 2} \\ {{{\ln \mspace{14mu} V_{2}} - {\ln \mspace{14mu} V_{1}}} = {C\left( {P_{2} - P_{1}} \right)}} & {{Equation}\mspace{14mu} 3} \\ {V_{2} = {V_{1} \cdot ^{C{({P_{2} - P_{1}})}}}} & {{Equation}\mspace{14mu} 4} \\ {{\Delta \; V} = {V_{2} - V_{1}}} & {{Equation}\mspace{14mu} 5} \end{matrix}$

When calculating the volume required to decompress the intake side, V₁ is the known dead volume (including flowline to check valves), P1 represents hydrostatic pressure and P2 is the measured flowing pressure at the sandface minus flowline loses. When calculating the volume required to compress the output side of the displacement unit, V1 the known total displacement unit volume (including flowline to check valves), P2 represents hydrostatic pressure and P1 is the measured flowing pressure at the sandface minus flowline losses.

The displacement unit volume change (ΔV) from Equation 1.

$\begin{matrix} {{Time} = \frac{\Delta \; V}{Q}} & {{Equation}\mspace{14mu} 6} \end{matrix}$

Combining equations 4, 5 and 6 the time may be calculated to compress/decompress the displacement unit:

$\begin{matrix} {{Time} = \frac{{V_{1} \cdot ^{C{({P_{2} - P_{1}})}}} - V_{1}}{Q}} & {{Equation}\mspace{14mu} 7} \end{matrix}$

If a compositional fluid analyzer is run, an equation of state can be populated to compute a fluid compressibility C. Compressibility can be calculated from a pressure density curve or compressibility can be measured by a sensor placed anywhere in the flowline.

Note that for multi-phase flow the compressibility of both phases (heaviest phase and lightest phase) should be taken into account for the output side. On the intake side the dead volume will be occupied by the lightest phase for intake in the upper cavity and by the heaviest phase for intake in the lower cavity.

In another embodiment, although the flow routing check valves are not shown in the FIGS., the flow routing check valves are present to route the fluid flow. Many different concepts may be used for such valves. The valves can be active or passive. After the displacement unit piston movement direction changes, the function of the check-valves changes from checking to flowing. Similarly, the flowing check-valves start to check flow. The time required to compress or decompress the displacement unit after a stroke direction change can also be determined by monitoring the check valve status. The time between the displacement unit direction change (known from piston position monitoring) and the check valve status change represents the time required to compress/decompress the displacement unit.

A flow detection sensor placed at the displacement unit intake and output may be used to calculate the time between each stroke-end and the start of flow into the displacement unit. Such a sensor would only have to differentiate “flow” from “no flow”.

Referring to FIG. 4, a method 400 to detect flow rate from a displacement unit is provided. In 402, a compressibility of a downhole fluid is measured. In 404, a dead volume for the displacement unit is calculated. In 406, a hydrostatic pressure and displacement unit outlet pressure for the displacement unit is measured. In 408 a flowing pressure of the downhole fluid at a sand face and displacement unit inlet is measured. In 410, a rate of displacement unit volume change is calculated and in 412, a time required to compress the displacement unit from the rate of displacement unit volume change is calculated.

The method 400 may be accomplished wherein the time required to compress the displacement unit is:

${Time} = {\frac{{V_{1} \cdot ^{C{({P_{2} - P_{1}})}}} - V_{1}}{Q}.}$

Referring to FIG. 5, a method 500 to detect flow rate from a displacement unit is illustrated. The method may accomplish, in 502, locating the displacement unit in a fluid path and in 504 monitoring a piston position in the displacement unit for a stroke direction change. In 506, a timer may be started upon a stroke direction change and in 508 at least one check valve position for an alteration of position may be monitored. Also included in the method 500 are, in 510, stopping the timer upon the alteration of position and in 512 calculating a time between the stroke direction change and the alternation of position. In 514, the method continues with calculating a volume of the displacement unit and in 516 calculating the flow rate from the displacement unit by dividing the volume of the displacement unit by the calculated time.

While the aspects have been described with respect to a limited number of embodiments, those skilled in the art, having benefit of the disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the disclosure herein. 

What is claimed is:
 1. A method to detect flow rate from a displacement unit, comprising: measuring a compressibility of a downhole fluid; calculating a dead volume for the displacement unit; measure a hydrostatic pressure for the displacement unit; measure a flowing pressure of the downhole fluid at a sand face; calculating a rate of displacement unit volume change; and calculating a time required to compress the displacement unit from the rate of displacement unit volume change.
 2. The method according to claim 1, wherein the time required to compress the displacement unit is: ${Time} = {\frac{{V_{1} \cdot ^{C{({P_{2} - P_{1}})}}} - V_{1}}{Q}.}$
 3. A method to detect flow rate from a displacement unit, comprising: locating the displacement unit in a fluid path; monitoring a piston position in the displacement unit for a stroke direction change; starting a timer upon a stroke direction change; monitoring at least one check valve position for an alteration of position; stopping the timer upon the alteration of position; calculating a time between the stroke direction change and the alternation of position; calculating a volume of the displacement unit; and calculating the flow rate from the displacement unit by dividing the volume of the displacement unit by the calculated time.
 4. The method according to claim 1, wherein the displacement unit is located in a wellbore.
 5. The method according to claim 3, wherein the displacement unit is located in a wellbore. 